Hyperfine structure of EPR spectra of Cr5+ ions in irradiated sodium dichromate crystals

EPR spectra are studied of X-irradiated sodium dichromate crystals grown from an aqueous solution by evaporation at 31°C. Doublet lines of EPR-absorption are attributed to the Cr⁵⁺ ions in the CrO₄³⁻ and CrO₃⁻ radicals resulting from radiation decomposition of Cr₂O₇²⁻ and being in the lattice in two unequivalent positions. Hyperfine structure caused by interaction of an unpaired electron with Cr⁵³ nucleus were observed both at liquid nitrogen and room temperatures. For the line, caused by CrO₄³⁻ radical, g_y and A_y directions coincide and angles both between A_x, g_x and A_z, g_z make up ∼ 25°. The spectrum is described by usual spin Hamiltonian for S = 1/2 with following parameters (T = 77 K): for CrO: g_z = 1.984, g_y = 1.970, g_x = 1.961, |A_z| = 8.2 · 10⁻⁴ cm⁻¹, |A_y| = 13.7 × 10⁻⁴ cm⁻¹, |A_x| = 36.1 · 10⁻⁴ cm⁻¹; for CrO₃⁻: g_z = 1.915, g_y = 1.975, g_x = 1.985, |A_z| = 32.2 · 10⁻⁴ cm⁻¹.     

Plastische Verformung von Siliziumeinkristallen unterschiedlicher Ausgangsversetzungsdichte im Streckgrenzenbereich

Floating-zoned Silicon crystals orientated for single slip both dislocation-free and with grown-in dislocations (EPD₀ = 6,5 · 10⁴ cm⁻²) were dynamically deformed in compression in the temperature range 1120 … 1300 K up to the lower yield point. The dislocation structure of plastically deformed crystals (T = 1138 K; a₀ = 1,45 · 10⁻⁴ s⁻¹) was examined by etching and high-voltage electron microscopy. The crystals exhibit essential differences in deformation behaviour and in the resulting dislocation structure. Typical for the dislocation-free basic material are (i) an inhomogeneous deformation in the range of the upper yield point (ii) a greater portion of sessile lomer dislocations at the lower yield point in relation to the crystal with grown-in dislocations. The measured stresses were related to those values calculated from structure data by employing plasticity theory of diamond-like semiconductors.     

Electron Spin Resonance of Mn2+ in Hemimorphite

The Electron Spin Resonance of Mn²⁺ in natural crystals of hemimorphite was studied at RT, X-band. No fine structure other than the central M = | + 1/2 〉 ⇆ | −1/2 > transition with allowed (Δm = 0) and forbidden (Δm = ±1) hyperfine transitions were observed. The spectrum was fitted with the spin Hamiltonian parameters g = 2.001, A = −84.6 × 10⁻⁴ cm⁻¹. The crystal field parameter D was estimated equal to (8 ± 2) × 10⁻³ cm⁻¹. The most striking result is the size of the hyperfine splitting constant favoring occupancy of sixfold coordinated sites.     

X-ray diffraction study and quantitative measurements of high dislocation densities in superlattices and single crystal layers of heteroepitaxial systems with pronounced lattice mismatch

Plastic deformation in a single-crystal layer of the In_0.12Ga_0.88As/(111)InP solid solution is identified by the methods of X-ray diffractometry (XRD) and the double-crystal pseudorocking curves (DCPRC). X-ray topographs showed the generation of three intersecting systems of straight dislocations in the layer. In a one-layer ZnSe/GaAs structure and multilayer ZnSe/ZnSe_{1 − x} S_x/ZnSe/GaAs structures, the elastic and plastic strains were detected by the combined XRD-DCPRC method. The major components of the thermoelastic and plastic-deformation tensors were determined as ε_xx = ε_yy = 3.5 × 10⁻³ and ε_zz = 2.35 × 10⁻³. Using these data, the dislocation densities were determined as N _d ∼ 2.5 × 10⁸ cm⁻² and N _d ∼ 3 × 10¹⁰ cm⁻² for the 7 μm-thick ZnSe and 1 μm-thick InAs layers, respectively. In a superlattice of the Al_xGa_{1 − x} As/GaAs/⋯/GaAs-type with a large lattice parameter, the plastic deformation was detected. X-ray topography confirmed that the dislocation density in this superlattice equals ∼10⁵ cm⁻². __________ Translated from Kristallografiya, Vol. 45, No. 2, 2000, pp. 326–331. Original Russian Text Copyright ? 2000 by Kuznetsov.     

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity ^tBuSH, ^tBu₂Se and the adduct Me₂Zn:Et₃N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm².     

Investigations for Sn diffusion in Pb1−xSnxTe and PbTe

By annealing Pb_1−xSn_xTe and PbTe isothermally in a quartz ampoule Sn diffused from Pb_1−xSn_xTe into PbTe. The profiles obtained have been investigated by means of an electron beam microanalyser, and the coefficients of diffusion have been determined at various temperatures. The diffusion of Sn can be explained by the expressions: D_PbSnTe = 1.5 · 10⁻¹ exp (−1.8 eV/kT) cm² s⁻¹ (0,14 < x < 0,18) D_PbTe = 5,5 · 10⁻⁴ exp (−1.5 eV/kT) cm² s⁻¹. N-type layers are observed at the surface of Pb_1−xSn_xTe specimens.     

The growth of HgI2 crystals

The paper presents a system for αHgI₂ crystal growth by the temperature oscillation method. The system has a capability of crystal growing at an excess I₂ or Hg vapour pressure. Optimum conditions for producing crystals up to 2 cm³ by volume have been established. The crystals grown at an excess I₂ vapour pressure have higher resistivity and higher drift electron and hole mobilities — μ_e = 120 cm² V⁻¹ s⁻¹ and μ_h = 6 cm² V⁻¹ s⁻¹, respectively.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

Kinetics of sodium fluosilicate precipitation

The formation of Na₂SiF₆ by discontinuous precipitation of dilute H₂SiF₆ with a 40% excess of an aqueous solution of NaCl under various conditions was studied. The values of induction time, number of crystals formed, their final size and their habit were determined during precipitation from solutions, whose initial supersaturation was 2 < S < 11. At S < 7–8 the crystals were formed by heterogeneous nucleation, whereas at S ≳ 7–8 homogeneous nucleation mechanism began to prevail. Once formed, the Na₂SiF₆ crystals were growing according to the screw-dislocation mechanism till they reached visible size; the corresponding values of kinetic order of nucleation and of the growth rate constant were g = 1.35 and k_g = 4.32 × 10⁻⁸ cm^2.05 sec⁻¹ g^−0.35, resp. The value of interfacial tension on the phase boundary Na₂SiF₆ crystal — saturated solution was determined (σ ∼ 52 erg/cm²). The resulting Na₂SiF₆ crystals conformed to log-normal distribution irrespective of conditions of precipitation. The dependence of the final size of crystals on supersaturation exhibited a maximum at S ∼ 6. Crystals of Na₂SiF₆ had a hexagonal habit, which was near to a spherical form at lower supersaturations, while dendritic crystals were formed at higher supersaturations.     

Crystal structure determination of 2-acetylamino-4-benzoyl-5,5-dimethyl-4,5-dihydro-1,3,4-thiadiazolin,C13H15N3O2S

Thiosemicarbazone is receiving increasing interest because of the biological activity of some of its derivatives. Mr = 277.35, space group P2₁2₁2₁, Z = 4, a = 9.290(3), b = = 11.512(2), c = 13.286(2) Å, V = 1421 (1) ų, D_m = 1.29(1) Mg m⁻³, D_x = 1.296 Mg m⁻³, λ(MoK_α) = 0.71062 Å, μ = 0.230 mm⁻¹, F(000) = 584, T = 296 K, final R = 0.028 for 1255 reflections. All hydrogen atoms were located. The structure determed by X-ray analysis confirms the chemical results. The structure analysis of the title compound is described and its molecular and crystal structure discussed.     

Thermoelectric power of indium sesquisulphide single crystals

The investigation covers a temperature range from 200 to 450 K. Thermoelectric power measurements of In₂S₃ crystals showed that all samples under investigation have a positive TEP in all temperature ranges, indicating n-type conductivity for In₂S₃ crystals. The ratio of the electron and hole mobilities is μ_n/μ_p = 4.71. The effective mass of electrons m is found to be 0.00008 × 10⁻³¹ kg. The obtained effective masses of holes m = 1.893 × 10⁻³¹ kg. The diffusion coefficient for both carriers (electrons and holes) is evaluated to be 84.71 cm²/s and 17.985 cm²/s respectively. The mean free time between collision is estimated to be τ_n = 1.7 × 10⁻²⁰s, and τ_p = 8.5 × 10⁻¹⁷s. The estimated diffusion length for electrons is found to be L_n = 1.2 × 10⁻⁹ cm and L_p = 3.9 × 10⁻⁸ cm.     

EPR-spectrum of irradiated potassium dichromate single crystals

EPR-spectra of X-irradiated potassium dichromate crystals are studied at various temperatures from RT to LNT. The most intensive lines are interpreted as due to radicals CrO₄³⁻ (A-lines) and CrO₃⁻ (C-lines), resulting from Cr₂O₇ owing to X-irradiation. Hyperfine structure of C-line, due to interaction of unpaired electron with Cr⁵³ nucleus, occurs at low temperatures, and new unobserved earlier Q-lines appear near-by C-line. The spectrum may be described by usual spin Hamiltonian for S = 1/2 with following parameters: for A-line gz = 1.9841, gy = 1.9685, gx = 1.9592; for C-line gz = 1.9148, gy = 1.9671, gx = 1.9886, |Az| = 30.53 · 10⁻⁴ cm⁻¹, |Ax| = 10.62 · 10⁻⁴ cm⁻¹, |Ay| = 6.62 · 10⁻⁴ cm⁻¹.     

Preparation and some properties of thallium orthothioarsenate single crystals

Growing of large (Ø 25–30 mm, L = 50–60 mm) optically homogenous single crystals of Tl₃AsS₄ using the Bridgman-Stockbarger method is described. Tl₃AsS₄ is orthorhombic (Pnma), a = 8.85 ± 0.03, b = 10.86 ± 0.03, c = 9.18 ± 0.03 Å; z = 4; ϱ_x = 6.15 g · cm⁻³; ϱ_p = 6.15 ± 0,03 g · cm⁻³, T_m = 424 ± 3°C, Moh's hardness = 3, microhardness and ultrasonics velocity along x, y, z are 65–85 kg · mm⁻² and 2.16 – 2.37 · 10⁵ cm · sec⁻¹, respectively. The crystals possess perfect cleavage plane (010). Their transparency range is 0.6–12 microns. – Unsuccessful attempts to obtain Tl₃AsS₄ and its alloys in the vitreous state were taken. The possibilities of glass formation and boundaries of the vitreous region in the system Tl–As–S based on the characteristic features of the melt forming structural units are analyzed.     

Investigation of radiation defects in electron irradiated Hg1−xCdxTe crystals using positron annihilation

The electron characteristics of defects in the initial and electron irradiated Hg_1−xCd_xTe (2–3 MeV, 10¹⁸ cm⁻², 300 K) crystals using the positron annihilation method have been investigated. The data of electric measurements are confirmed on connection of p-type conductivity with vacancy defects of metal sublattice initial crystals Hg_1−xCd_xTe. An analysis of correlation curves of irradiated crystals has shown a possibility of formation of associations of initial defects and radiation damages of vacancy type during radiation process. The presence of narrow component on correlation curves in the region of small angles is associated with formation of positronium states localized in the region of radiation defect complex of vacancy type. Identification of positron-sensitive defects with electrically active radiation induced ones has been carried out according to the results of isochronal annealing of irradiated crystals.     

On the Density of Synthetic Glasses in the System Soda Melilite – Gehlenite/Ákermanite

The solid density has been determined at 25°C by hydrostatic weighing along the binary systems Sm–Ge₅₀Åk₅₀ and Sm–Ge₇₀Åk₃₀. The molar volume V decreases linearly with increasing mole fraction of Ge_mÅk_n, x_B, according to δV/δx_B = −5.08 cm³ mol⁻¹ and −4.40 cm³ mol⁻¹ for Ge₅₀Åk₅₀ and Ge₇₀Åk₃₀, respectively. The results enable the molar fraction x_B to be determined from density measurements with an absolute error of ±5 mol%.     

Crystal and Molecular Structure of Tosyl Pyrrolidine-2-Methanol (C12H17SO3N)

C₁₂H₁₇SO₃N, M_r = 255.33, Orthorhombic, P2₁2₁2₁, a = 11.703(1) Å, b = 14.797(3) Å, c = 14.971(2) Å, V = 2592.52 ų, Z = 8, D_m = 1.309 Mgm⁻³, D_c = 1.308 Mgm⁻³, mμ = 21.57 cm⁻¹, F(000) = 1088, T = 290 K, final R = 0.080 for 2416 unique reflections. There are two crystallographically independent molecules in the unit cell of the title compound.     

Crystal structure of a newly oxidized aristolane sesquiterpenes,C15H22O3, from aristolochia debilis

M_r = 250, orthorhombic, P 2₁2₁2₁, a = 9.583(6), b = 11.786(8), c = 12.298(9) Å, V = 1389.0 ų, Z = 4, D_x = 1.20 g cm⁻³, F(000) = 544, MoK_α, λ = 0.71069 Å, μ = 0.88 cm⁻¹. Final R = 0.023 for 1006 observed reflections, T = 293 K. The structure was solved by direct methods and refined by full-matrix least squares. It is an α, β unsaturated carbonyl compound with a trisubstituted double bonds. No significant variation in aromatic bond lengths. The strain affecting the molecule is largely due to angular deformation which is partly due to the fusion of the rings and partly due to substitution of H-atoms by methyl groups and oxygen atoms.     

Effect of doping and heat treatment on the mechanical parameters of ZnSe<Subscript>(1 − <Emphasis Type="Italic">x</Emphasis>)</Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> crystals grown from the melt

The mechanical properties of crystals of ZnSe_{(1 − x)}Te _x (0 < x < 1.3 wt %) solid solutions are investigated using the indentation method. The breaking points of doped and undoped crystals are measured by the uniaxial compression method. It is revealed that the microhardness anisotropy coefficient for crystals with a tellurium dopant content of ∼0.3 wt % is equal to unity. A change in the tellurium content in the solid solutions from 0.2 to 1.3 wt % leads to a linear increase in the microhardness by 23%. The brittle strength of the ZnSe and ZnSe_{(1 − x)}Te _x crystals varies in a similar manner. It is demonstrated that heat treatment and the presence of interblock boundaries affect the ultimate strength and the cracking resistance of the ZnSe_{(1 − x)}Te _x crystals. This is an important factor which should be taken into account in mechanical treatment of the materials under investigation.     

Crystal and Molecular Structure of 1-Ethyl-3-[tris(trimethylsiloxanyl)silyl]pyrrolinium Hydrochloride

The crystal and molecular structure of 1-Ethyl-3[tris(trimethylsiloxyl)silyl]pyrrolinium hydro-chloride (C₁₅H₃₈N⁺O₃Si₄ · C¹⁻) has been determined by direct methods. The title compound crystallizes in the monoclinic space group C2/c with a = 20.640(3), b = 19.494(2), c = 27.34(3) Å, β = 90.60(4)°, V = 11000(13) ų, Z = 16, D_x = 1.034 Mg m⁻³. There are two molecules with different conformations in the crystal. The pyrroline rings are non-planar.-The Si O Si angles range from 149(1)° to 163(1)°. Two of the SiMe₃ groups are disordered. All molecules are connected by C¹⁻ – N⁺ contacts and C¹⁻ - HN⁺ hydrogen bonds to form double chains.     

Electrical Characterization of Irradiation plus Annealing – induced VAsZnGa Pairs in p-type GaAs (Zn) Crystals

Effect of fast electron irradiation (E =2.2 Mev, ϕ_c = 1 × 10¹⁶ el/cm²) and subsequent annealings (T = 150 to 350 °C, t = 10 to 600 min) of zinc-doped p-type GaAs crystals on the formation and dissociation of V_AsZn_Ga, pairs is studied. An analysis of the formation and dissociation kinetics of V_AsZn_Ga pairs permitted to find the diffusion coefficient of radiation-induced arsenic vacancies D(D = 1.5 × 10⁻¹⁸, 1 × 10⁻¹⁷ and 5 × 10⁻¹⁷ cm²/s at 150, 175 and 200 °C accordingly), their migration energy ϵ_m(ϵ_m = 1.1 eV), the binding energy of V_AsZn_Ga, pairs ϵ_b(ϵ_b = 0.5 eV), and also their dissociation energy ϵ_d(ϵ_d = 1.6 eV).